1. Field of the Invention
The present invention relates to poly gate fabrication, and more particularly to a method for forming the metal silicide layer within poly gate.
2. Description of the Prior Art
Polysilicon is generally used to form the conductive part of metal-oxide-semiconductor (MOS), but the resistance value of polysilicon is too high to use even it is doped with impurity. Therefore, metal silicide is generally formed on polysilicon and used to decrease the resistance value of conductive part of MOS and then the operation of gate is improved. In addition, if there is any thermal processes in the following process of gate fabrication, phase transformation of the metal silicide may occur and the structure of gate is changed. No matter how, the ratio of metal to silicon is variable, which depends on the chemical reaction and the chamber condition, etc. Thus, the phase transformation temperature is sensitive depending on the ratio of metal to silicon.
The known process of poly gate formation comprises following steps. First, as shown in FIG. 1A, forming a dielectric layer 110 over a substrate and then forming a polysilicon layer 112 on dielectric layer 110. A metal silicide layer 114 is then formed on the polysilicon layer 112, where the popular metal silicide is tungsten silicide. Afterwards, as shown in FIG. 1B, a passivation layer 120 is capped over the metal silicide layer 112, since the capping process is a high temperature process so the structure of metal silicide is transformed to a new metal silicide structure 116. Furthermore, an anti-reflection layer 118 could be formed over metal silicide layer 116 before forming the passivation layer 120. Subsequently, conventional standard photolithography method is used to form the primary structure of poly gate as FIG. 1C shows. After the etching process is over, gate etch anneal process is used to restore the etching damage on the surface and sidewall oxide 124 is formed on the sidewall to protect the poly gate as shown in FIG. 1D. It is obvious that because both gate etch anneal and sidewall oxide formation are high temperature process, the structure of metal silicide 116 maybe change during these process and another metal silicide structure 122 is formed. The metal silicide structure 122 replaces the previous metal silicide structure 116 and induces peaks, dips and nodules on the surface of poly gate. In other words, the surface of poly gate is roughened and those extrusions often cause some disadvantages. For example, when a poly gate is close to other gate then these extrusions in different poly gate maybe is too close so current is conducted from one poly gate to another poly gate, this is so-called gate to gate short.
By the way, it is necessary to improve the fabrication of poly gate such that these extrusions vanish and the structure of metal silicide layer is stationary during any following treatment of poly gate.